Electro-optical measuring devices with a bi-axial system of transmission and reception path are used, e.g., as contactless distance measuring devices, and they have been commercially available as such for quite some time. These devices emit a modulated measurement signal via a transmission path. The modulated measurement signal is reflected and/or scattered on the surface of a target object whose distance from the device is to be determined. A portion of the reflected or scattered measurement radiation returns to the measuring device, where it is detected by a receiver unit.
The distance of the target object from the measuring device may be deduced from the transit time of the measurement signal and/or from the phase shift between the transmitted measurement signal and the detected, returning measurement signal, which is correlated with the transit time.
Bi-axial measuring systems, with which the transmission path is located a distance away from the reception path, have the advantage that a complex radiation-splitting system is not required to select the returning measurement signal, thereby enabling, e.g., optical crosstalk from the transmission path directly into the reception path to be suppressed to a greater extent.
Bi-axial measuring devices also require measures to compensate for the parallax between the transmission path and the reception path.
Although the depiction of the target object on the detector surface of the receiver is clearly located on the receiver even when the target distances are great, the image moves increasingly further away from the optical axis of the reception path as the measurement distance decreases, and the beam cross-section varies in the receiver plane.
As a result, the measurement signal may approach zero in the short range of detection, i.e., when the distance between a desired target object and the measuring device is short, if no further measures are taken in the device.
Various measures for parallax compensation of bi-axial measuring systems are known from the related art, some of which are based on the design of the receiver, and others of which are based on the design of the receiving lens system.
Publication DE 43 16 348 A1 makes known a device for distance measurement with a visible measurement beam created using a semiconductor laser, the reception device of which includes an optical waveguide with a downstream opto-electronic converter. The light input surface into the fibers of the optical waveguide is located in the plane of projection of the receiving lens of this device for large object distances, and it is displaceable out of this position, transversely to the optical axis. In this manner, the device described in DE 43 16 348 A1 makes it possible to direct the measurement beams—that strike the receiving lens at increasingly greater slants as the object distances decrease—to the photosensitive surface of the detector by adjusting the optical fibers, if it is not possible to change the position of the detector.
As an alternative, to solve the parallax problem associated with bi-axial measuring devices, publication DE 43 16 348 A1 provides that the optical waveguide input surface is fixed in position, and that the imaging position of the measurement beam is deflected to the optical axis of the receiving lens using optical deflection means when shorter object distances are involved.
To this end, publication DE 43 16 348 A1 provides that prisms or diffractive optical elements are used, which are located near the receiving lens.
Publication DE 198 60 464 A1 makes known a laser rangefinder for large measuring ranges with parallel transmission and reception channels, in which case the receiving lens is a modified lens element composed of a primary lens region with a primary optical reception axis that is oriented parallel to the optical transmission axis, and with a secondary lens region with a secondary optical reception axis, which is tilted by an angle α relative to the primary optical reception axis, thereby resulting in a primary focal point and a secondary focal point.
Publication WO 92/05455 makes known a coaxial, optical distance measuring device, the receiving lens system of which include zone-type elements having different focal distances. Using this special receiving lens system, light from different object distances is focussed on the receiver in an optimal manner by the assigned zonal element of the imaging lens system.